In industrial finishing applications, coating particles are emitted from a spray device such as a spray gun toward an object to be coated. One type of coating material is in the form of particulate powder entrained in a stream of air. The process of spraying products with a solid powder coating involves preparing the powder coating in finely ground form and spraying it onto the parts in a manner similar to liquid paint. Conventionally, but not necessarily, an electrostatic charge is supplied to the powder as it is sprayed toward an object to be coated. The object to be coated is maintained at an electrostatic potential different than that from the charged powder particles so that the particulate powder material is attracted to the article and deposited thereon with approved efficiency and coverage. The electrostatic charge maintains the powder on the product for a sufficient time period to permit the powder to be heated so that it melts, and when the powder is subsequently cooled it is firmly attached to the target substrate.
Powder spray guns generally comprise a barrel formed with a powder flow passageway and a spray nozzle mounted at the forward end of the barrel. The spray nozzle may be formed with a generally circular-shaped discharge opening through which powder coating particles are emitted to form a generally conical-shaped spray pattern upon an object to be coated. Alternatively, the spray nozzle is formed with a generally rectangular-shaped slot through which the powder coating particles are discharged to form a so-called "flat" spray pattern, i.e., a spray pattern with relatively sharply defined, parallel edges. An electrode is often mounted at the forward end of the spray gun, in the vicinity of the discharge opening in the spray nozzle, to impart an electrostatic charge to the powder coating particles emitted from the spray nozzle.
In order to maximize coverage of a target object with coating particles, it is desirable to create a strong electrostatic field between the electrode and target object so the coating particles are efficiently charged and then strongly attracted to the target object. It has been found that this electrostatic field is strengthened by positioning the electrode forwardly of the spray nozzle and the discharge opening therein.
In prior art "flat" spray nozzles, i.e., those having a rectangular discharge opening, the electrode has been positioned forwardly of the spray nozzle but this has resulted in undesirable interaction between the pattern forming structure and charging structure with a resulting loss of performance in one or both. For example, if the electrode is positioned in the path of particulate powder material emitted from the slotted discharge opening in order to increase charging efficiency, the spray pattern on the object to be coated is disrupted. Movement of the electrode out of the path of the powder material, as disclosed, for example, in U.S. Pat. No. 4,630,777, avoids disruption of the spray pattern but the charging efficiency decreases to some degree.
Another approach in prior art electrostatic spray guns employing a flat spray nozzle has been to position the electrode within the interior of the nozzle. This avoids disruption of the spray pattern, but substantially weakens the electrostatic field between the electrode and target object because the electrode is shielded by the nozzle. Additionally, where the electrode is positioned inside of the spray nozzle, an ignition hazard can be created by a sudden capacitive discharge between the electrode and a grounded object. Whereas an electrode positioned outside of the spray nozzle forwardly of the gun gradually discharges as a grounded object approaches, an electrode positioned internally of the spray nozzle may be partially or completely shielded from an approaching grounded object. As a result, the grounded object can "sneak up" on the electrode, i.e., approach the electrode while being shielded by the nozzle, and thus cause a sudden, relatively high energy capacitive discharge from the electrode to the grounded object. A high energy capacitive discharge can result in a spark or arc between the electrode and grounded object and ignite the volatile, potentially explosive environment in which powder spray guns are operated.
Another problem with flat spray nozzles employing a single, rectangular-shaped discharge slot or opening is that the spray pattern on the target object is often non-uniform. In many instances, the center or middle of the pattern is "heavy", i.e., has a concentration of particulate powder material. Alternatively, or in addition to a concentration at the center of the pattern, the edges of the pattern in prior art flat spray nozzles often do not decrease uniformly to allow for overlapping of patterns from adjacent spray guns without a "striping" effect, i.e., a buildup of powder at the overlapping edges.